The present invention relates to automatic equalizers which compensate for the distorting effects of bandlimited channels on transmitted data signals.
Automatic equalizers are necessary for accurate reception of high speed data signals transmitted over bandlimited channels with unknown transmission characteristics. The equalizer, which is resident in the receiver portion of a data set, or "modem", is generally in the form of a transversal filter. Samples of the incoming data signal, referred to herein as "line samples", are formed at a predetermined sampling rate. These are applied to the filter, where they are multiplied by respective tap coefficients. The resulting products are added together and, if necessary, demodulated to generate what is hereinafter referred to as a "basband equalizer output," or just "equalizer output." The equalizer output is thereafter quantized to recover the transmitted data. In addition, an error signal is formed equal to the difference between the equalizer output and a reference signal which represents the transmitted data symbol. In the so-called adaptive type of automatic equalizer, in particular, the reference signal is derived from the decision made in the receiver (on the basis of the equalized signal value) as to what data symbol was transmitted. The error signal is used to update the tap coefficient values in such a way as to minimize a measure of the distortion--assumed to be primarily intersymbol interference--introduced by the channel.
An important equalizer operating parameter, in addition to the rate at which the line samples are formed, is their time occurrence with respect to the received signal. This parameter, referred to as the timing epoch, is the principal focus of the present invention. In particular, equalizing a given channel when the line samples are taken at different sets of time points, i.e., with different timing epochs, results in different ensembles of tap coefficient values. Before accurate data recovery can be assured, then, it is necessary to arrive at an appropriate combination of timing epoch and coefficient values.
In steady-state operation, adaptive equalizers are typically capable of operating satisfactorily over at least a range of timing epochs. There is, however, a nominally optimum timing epoch, corresponding to a particular set of sampling points on the received signal. The optimum timing epoch, in particular, is that epoch which allows the channel to be "learned" most quickly. In typical equalizer start-up operation, hereinafter referred to as a "long" start-up, a timing acquisition tone from which this optimum timing epoch (or a close approximation thereto) can be ascertained, is sent ahead of the actual intelligence to be transmitted, the latter being hereinafter referred to as the "message" data. The timing acquisition tone is typically followed by a predetermined equalizer "training sequence," in response to which the coefficients converge, i.e., take on an ensemble of values which, for the selected timing epoch, corrects for intersymbol interference in the channel.
The long start-up approach is suitable for use in applications in which the transmitted messages are long compared to the start-up period. However, in some applications, such as many multipoint network applications, this condition is often not met. A multipoint network, more particularly, is comprised of a master, or control, modem connected to a plurality of slave, or tributary, modems via respective dedicated transmission channels. Each tributary modem receives data only from the master modem and thus over one particular channel. Accordingly, the tributary is able to continually use the same tap coefficient values to recover successive messages transmitted to it. In fact, each tributary modem is able to continually fine tune its coefficient values and timing epoch. This is because all transmissions emanating from the master modem are received (although not necessarily responded to) by each tributary modem over its channel from the master modem.
However, before the control modem can recover data from a particular tributary, its tap coefficients and timing epoch must be set to appropriate values for the channel associated with that tributary. Conventional start-up techniques, if used in multipoint network applications, would waste a great deal of valuable transmission time because the control modem typically receives communications from a particular tributary for only a short time before turning its attention to another. Indeed, conventional start-up techniques imose an upper limit on the throughput of such a system, i.e., the amount of message data which can be transmitted per unit time. This is because, in general, the higher the data rate the system is operated at, the longer the required start-up period. In order to ameliorate this problem, it has been proposed to store in the control modem the already-learned, or "converged", tap coefficients associated with each channel. When data are to be received over a given channel, the associated coefficient values are read out of memory and "jam set" into the equalizer, obviating the need for the modem to "relearn" the channel for each transmission.
Determining the appropriate timing epoch for the jam set coefficients presents a problem, however. In theory, a timing epoch could be established at the start of an initial message from a tributary in any of several ways. Once a set of tap coefficients is arrived at using this timing epoch, all one would need to do in order to use the same coefficients for subsequent transmissions from the same tributary would be to re-acquire the same timing epoch.
The problem with such an approach is that there is a tendency for the relative phase between the transmitter and receiver clocks to drift over time. This necessitates the use of some form of continually operating timing recovery circuitry, the function of which is to advance or retard the receiver timing circuitry so as to ensure that the received signal is, in fact, sampled with the correct timing epoch. If an ideal timing recovery technique were available, the above-suggested approach could, at least in theory, be workable in a system which uses coefficient jam-setting. As a practical matter, however, the timing recovery techniques of which we are aware are themselves subject to a certain amount of jitter--at least when operating on random data. That is, they are capable of maintaining the timing epoch only within some range about the nominally correct value. This is not a problem so far as accurate data recovery is concerned; as long as the rate of timing drift is within design limits, the tap coefficient updating algorithm will change the coefficient values to compensate for same. By the same token, when a previously-determined ensemble of coefficient values is jam set into the equalizer at the start of transmission, those coefficients will, in general, have different values than they had at any particular time at which the timing epoch was known during any previous transmission. They thus require a different, unknown, timing epoch. One way around this problem is to start with a predetermined timing epoch and allow the jam set coefficient values to rotate (in response to a training sequence, for example) to compensate for any timing epoch error prior to the transmission of message data. This, however, may be too time consuming. Alternatively, the sampling phase drift problem might be avoided by using very accurate or slaved clocks, eliminating the need to recover timing information from the received signal. This approach, however, is costly, complex and possibly unworkable.
A more advantageous solution is provided by the technique disclosed in U.S. Pat. No. 4,245,345 issued Jan. 13, 1981 to R. D. Gitlin et al, hereby incorporated by reference. In accordance with that technique a periodic timing acqusition signal is transmitted over the channel in question. The timing acquisition signal, after transmitter filtering, has spectral (frequency) components only within that portion of the equalized baseband-equivalent transfer function known as the non-rolloff region. The received timing acquisition signal is sampled with an arbitrary timing epoch, is equalized using a previously-determined ensemble of tap coefficient values for the channel and, unless at baseband, is demodulated. The resulting equalizer outputs, referred to as timing acquisition equalizer outputs, represent successive samples of a periodic waveform. The Gitlin et al patent teaches that if, and only if, the frequency spectrum of the periodic timing acquisition signal is limited to the non-rolloff region, the difference between (a) the locations of the above-mentioned samples along the periodic waveform and (b) what their locations therealong would be if the timing epoch were correct (which locations are known a priori), is equal to the timing error, i.e., the difference between the arbitrary timing epoch with which the line samples are currently being formed and the correct timing epoch. The timing error is readily determined from the timing acquisition equalizer outputs and is illustratively determined as a trigonometric function of as few as two of them. Once the timing error is determined, the timing epoch can be immediately adjusted to the correct value.